Heretofore, various type fuel injectors and fuel injection systems have been known in the prior art which are applicable to internal combustion engines. Of the many types of fuel injection systems, the present invention is directed to unit fuel injectors, wherein a unit fuel injector is associated with each cylinder of an internal combustion engine and each unit injector includes its own drive train to inject fuel into each cylinder on a cyclic basis. Normally, the drive train of each unit injector is driven from a rotary mounted camshaft operatively driven from the engine crankshaft for synchronously controlling each unit injector independently and in accordance with the engine firing order.
Of the known unit injectors of such fuel injection systems, there are two basic types of unit injectors which are characterized according to how the fuel is metered and injected. A first type of which the present invention is a modification is known as an "open nozzle" fuel injector because fuel is metered to a metering chamber within the unit injector while the metering chamber is open to the engine cylinder by way of injection orifices. Moreover, open nozzle injectors typically include a plunger assembly with a tip portion that seats around the injection orifices after injection.
In contrast to the open nozzle type fuel injector, there are also unit fuel injectors classified as "closed nozzle" fuel injectors, wherein fuel is metered to a metering chamber within the unit injector while the metering chamber is closed to the cylinder of an internal combustion engine by a biased valve mechanism that is opened only during injection by the increasing fuel pressure acting on the valve mechanism and overcoming the valve bias. Typically, the valve mechanism is a spring biased needle type valve.
In either case, the unit injector typically includes a plunger element that strikes the metered quantity of fuel to increase the pressure of the metered fuel and force the metered fuel into the cylinder of the internal combustion engine. In the case of a closed nozzle injector, a tip valve mechanism is provided for closing the injection orifices during metering wherein the tip valve is biased toward its closed position to insure that injection will take place only after the fuel pressure is increased sufficiently to open the tip valve mechanism against the bias force.
A known manner of supplying fuel to unit fuel injectors which is applicable to the present invention is a unit injector fuel injection system that relies on pressure and time principles for determining the quantity of fuel metered for each subsequent injection of each injector cycle Moreover, the pressure-time principles allow the metered quantity of fuel to be varied for each cyclic operation of the unit injector as determined by the pressure of the fuel supplied to the metering chamber and the time duration over which such metering takes place.
Examples of unit injectors of the open nozzle type are described in detail in U.S. Pat. Nos. 4,280,659 and 4,601,086 to Gaal et al. and Gerlach, respectively, both of which are owned by the assignee of the present invention. The injectors of Gaal et al. and Gerlach include a plunger assembly with a lower plunger having a major diameter section that is slidable within an axial bore of the injector body and a smaller minor diameter section that extends within a cup of the injector body. The cup provides an extension to the axial bore which is smaller in diameter than the diameter of the axial bore that passes through the remainder of the injector body. During the metering stage of the Gaal et al. and Gerlach injectors, fuel is metered through a supply port into the axial bore at a point above the cup, and the fuel flows around the minor diameter section of the plunger assembly at the tip thereof for metering a specified quantity of fuel into the metering chamber of the cup. A radial gap is provided between the minor diameter section of the plunger assembly and the inner wall of the bore within the cup. This gap facilitates the flow of fuel to the injector tip to be injected. Once the metering stage is completed, the plunger travels inwardly (defined as toward the engine cylinder of an internal combustion engine) so as to cause injection of the fuel from the metering chamber through the injection orifices.
A serious problem that is unique to open nozzle-type unit fuel injectors is the sensitivity of fuel metering to carboning of the unit fuel injector. Injector carboning occurs on all of the surfaces of the minor diameter section of the plunger and the inner surface of the cup. As best understood, the carbon forms as a result of essentially oil, fuel, and the temperature of the gases within the unit injector metering chamber. Moreover, carboning has a greater tendency to occur during certain engine operating conditions wherein little or no fuel is supplied to the metering chamber within an injector cycle. Such conditions include that which is defined as a motoring condition where the engine is being driven from the vehicle drive train. During motoring, the plunger is lifted in accordance with the injector cycle as controlled by the associated camshaft, but little or no fuel is supplied. At the same time, the engine piston is experiencing a compression stroke, which pressurizes the cylinder gases and forces the hot gases back into the unit injector through its nozzle. The lack of fuel in the metering chamber during such a condition allows the gas temperatures inside the metering chamber to become very high.
Additionally, when the plunger tip unseats from the cup, airborne carbon enters the metering chamber from the engine combustion chamber through the injector spray holes. This airborne carbon then deposits on to the surfaces of the plunger and cup. A study of the carbon deposits on the plunger and cup has shown that, in cross section, a first layer of deposits on the surfaces is related to fuel and acts as a kind of adhesive. The outer layer consists of hard black carbon deposits which result mostly from oil. This outermost layer of deposits is responsible for creating this major problem of open nozzle-type unit injectors in that the deposits create injector flow loss which inhibits the flow of fuel into the metering chamber during metering.
During metering, fuel must be able to pass between the minor diameter section of the plunger and the inner wall of the cup so as to flow to the metering chamber at the cup tip. As the carbon deposits increase in thickness, the flow loss also increases. At some point it becomes impossible to obtain a sufficient fuel flow between the plunger minor diameter section and the cup inner wall such that a sufficient volume of metered fuel can be created for injection. At this point, the unit injector cannot function properly.
Thus, in order to deal with the carboning situation, it has become necessary to replace, or at least service, such open nozzle unit fuel injectors after a period of running time, depending on operating conditions. As an alternative, efforts have been concentrated on reducing the formation of carboning as a means of lessening the effect of carboning on injector flow metering. However, once carboning eventually builds up, the injector will inevitably experience some injector flow loss.
For the above reasons, the popularity of closed nozzle fuel injectors has increased; however, the immediate disadvantage associated with closed nozzle fuel injectors is the extra costs that are associated with the production of such substantially more complex unit fuel injectors. Apart from the fact that a closed nozzle unit fuel injector functions on different operational principles than an open nozzle injector, as amplified above, closed nozzle injectors do not experience the same problems of open nozzle injectors enumerated above. Specifically, the valve of the closed nozzle injector does not have to be designed to accommodate precise metering at the nozzle. Furthermore, injector carboning is not as prevalent in closed nozzle unit fuel injectors because the biased nozzle valve effectively closes the interior of the unit injector at the very tip thereof from the engine combustion chamber during motoring or the like conditions.
Other noteworthy U.S. patents with respect to the present invention are U.S. Pat. Nos. 3,831,846 to Perr et al. and 4,650,121 to Augustin. Perr et al. '846 is also owned by the assignee of the present invention and discloses an open nozzle type fuel injector including a tip valve that is movable with respect to the plunger assembly and located within the cup of the injector. The cup includes an enlarged bore, as seen in FIGS. 10-15, having an upper ledge retaining a biasing mechanism within the cup. The device, however, is provided as a means for preventing "secondary injection", and as such is subject to injector carboning the same as the above-described prior art open nozzle injectors. Augustin '121 discloses a closed nozzle unit fuel injector including an insert body 6 press-fit within a cylindrical opening of the injector body which includes swirl channels, wherein the nozzle valve is slidable within the insert body. This injector is not concerned with preventing injector carboning.
Thus, there is a need for a unit fuel injector which prevents injector carboning that occurs during certain engine operating conditions of the type with no fuel or part fuel injection requirements. Moreover, there is a need to provide such a unit fuel injector that is relatively simple to operate and easy to produce along the line of open nozzle unit fuel injectors but which incorporates the advantages of the more complicated closed nozzle injectors with regard to injector carboning. Preferably, such a unit injector will function accurately over the entire useful life of such a unit injector without suffering from excessive flow losses that result from carboning of the plunger and cup surfaces.